Liquid phase olefin oligomerization using catalysts consisting of a titanium or nickel complex and an alkylaluminum has been known. This oligomerization process, however, entails separating and recovering the catalysts and is thus complicated. Olefin oligomerization is also known to be performed by a liquid phase or gas phase process with use of catalysts in which nickel is supported on a support composed of silica, alumina, silica and alumina, or the like.
Patent Document 1 discloses catalysts wherein 0.1 to 5 wt % of nickel is supported on a support composed of silica and alumina (alumina content: 1 to 10 wt %). The silica/alumina molar ratio (SiO2/Al2O3) is calculated to range from 15 to 168. It is described therein that the catalytic activity is not enhanced if the alumina content is less than 1 wt %, namely, if the silica/alumina molar ratio (SiO2/Al2O3) exceeds 168.
Patent Documents 2 to 5 disclose catalysts obtained by coprecipitation of nickel, silica and alumina; catalysts obtained by supporting nickel on a silica/alumina support through impregnation cycles; and catalysts obtained by adding ammonium hydroxide to a nickel nitrate solution to obtain ammoniacal nickel and impregnating a silica/alumina support with the nickel. The nickel content in these catalysts is 2 wt % or more.
Patent Document 6 discloses catalysts wherein the nickel oxide/silica molar ratio is 0.001 or more and the silica/alumina molar ratio (SiO2/Al2O3) ranges from 30 to 500. The nickel content is described to be 0.092 wt % or more. Preferred embodiments and working examples disclose 0.021 as a nickel oxide/silica molar ratio, and the nickel content is calculated therefrom to be 1.7 wt %.
Further, the catalysts of Patent Documents 2 to 6 have a problem of short life under high temperature reaction conditions. Furthermore, the catalysts of Patent Documents 1 to 6 easily decrease catalytic activity and induce isomerization to afford branched oligomers of low utility value.
Patent Document 7 discloses that a catalyst supporting nickel on a silica/alumina support is reacted with a sulfur-containing compound to afford a catalyst which supports nickel and sulfur on the support. However, the catalysts supporting nickel and sulfur show inferior activity to catalysts which support nickel alone.
Non-Patent Document 1 discloses catalysts wherein nickel is supported on a silica/alumina coprecipitated support by ion exchange. The catalysts are described to catalyze ethylene oligomerization under conditions such that the ethylene conversion is high at 90% or more. However, such high conversion conditions lead to an increased molecular weight of the oligomer due to successive reactions, and it is impossible to obtain low molecular oligomers such as dimers. Further, the catalysts permit long operation at a reaction temperature of 108° C., but a slight increase in reaction temperature to 127° C. causes irreversible deactivation.    Patent Document 1: U.S. Pat. No. 2,581,228    Patent Document 2: U.S. Pat. No. 2,921,971    Patent Document 3: U.S. Pat. No. 2,949,429    Patent Document 4: U.S. Pat. No. 3,045,054    Patent Document 5: U.S. Pat. No. 2,904,608    Patent Document 6: WO 93/06926    Patent Document 7: U.S. Pat. No. 3,527,839    Non-Patent Document 1: J. Haveling, C. P. Nicolaides, M. S. Scurrell, Catalysts and conditions for the highly efficient, selective and stable heterogeneous oligomerization of ethylene, ELSEVIER, Applied Catalysis A, 1998 Vol. 173, pp. 1-9